<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>


    <style>
        .box {
            border: 1px solid red;
        }
        .da {
            float: left;
            width: calc(100% - 80px);
            height: 200px;
            background-color: green;
        }
        .xiao {
            float: left;
            background-color: hotpink;
            height: 100px;
        }
        .clear {
            clear: both;
        }
    </style>
</head>
<body>
    

    <div class="box">
        <div class="da">
            大毛
        </div>

        <div class="xiao">
            小毛
        </div>
        <div class="clear"></div>
    </div>
</body>
</html>